[time-nuts] Square to sine wave symmetrical conversion (part 2)

Tim Shoppa tshoppa at gmail.com
Sun Jul 26 22:21:32 UTC 2015


  We seem to have wandered into one of my favorite subjects, oscillators
that make very low distortion sine waves.

  I found this 1960 HP journal article to be most useful:
http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1960-04.pdf

  Best quote:"In fact, were it not for [amplifier] nonlinearity, it would
be impossible to build a simple oscillator with good envelope stability
[...] This is borne out by the fact that occasionally oscillators show up
in production which, because of a fortuitous combination of tube
characteristics, exhibit extremely low distortion. Invariably these units
give trouble with with envelope bounce [...]

N1EKV also had a nice article in a 2010 QEX about the battle between
amplitude stability and low distortion sine wave output (not sure this is
available online).

Tim N3QE

On Sun, Jul 26, 2015 at 2:48 PM, jerry shirᴀr <radio.n9xr at gmail.com> wrote:

> Hi Bob,
>
> Help me to understand your remarks.
>
> A.  The oscillator stage has a limiter.  (Agreed.)
> B.  ​You can *easily* get <-160 dbc/Hz with an oscillator that has
> harmonics in the
> -10 to -20 range?
> C.  But we are not talking about the signal from the oscillator stage
> itself?
> D.  Tuned buffers can have these higher harmonics and all is well?
> E.  Okay.  We are now talking about downstream harmonics and not the actual
> oscillator stage harmonics?
>
> Ideally the oscillator stage should have no tuned circuits so that the
> feedback of this oscillator stage utilizes only the tuned circuit of the
> high Q resonator.  Okay.  What frequency components should the hi Q
> resonator exhibit?  Should it be rich in harmonics or should it be pretty
> limited to one frequency represented by a sinewave?  Are we talking about
> the harmonics at the output of the actual oscillator stage or after a few
> buffer stages?
>
> Help me out here.
>
> Thanks,
>
> Jerry N9XR
>
>
> On Sun, Jul 26, 2015 at 8:44 AM, Bob Camp <kb8tq at n1k.org> wrote:
>
> > Hi
> >
> > Ummm …. errr….
> >
> > The oscillator loop has a limiter in it or it’s not going to work very
> > well.
> > ​​
> > You can *easily* get <-160 dbc/Hz with an oscillator that has harmonics
> in
> > the
> > -10 to -20 range. With some tweaking you can get into the 170’s.
> >
> > It’s not the limiting by it’s self, it’s *how* the limiting is achieved.
> >
> > In most cases the “oscillator output” you are looking at is not the
> signal
> > from
> > the oscillator stage it’s self. You are looking at a signal that has been
> > through
> > several (like tuned) buffers before you see it. The same narrow band
> tuned
> > stages
> > can be used to “clean up” harmonics on any signal. The old style rack
> mount
> > OCXO’s did a *lot* of this.
> >
> > Bob
> >
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